Detecting the Undetectable: The growing sophistication of fileless attacks

Attackers are constantly looking for new ways to evade detection. New malware techniques take advantage of operating system features to inject malicious code into memory or the operating system registry without leaving a file on the disk. Traditional detection tools that rely on analyzing disk files need to be augmented with memory analysis and memory protection mechanisms, behavioral analysis, and broader intelligence sharing.

A current trend in fileless malware attacks is to inject code into the Windows registry. Most of these attacks enter a system as a file or link in an email message; this technique serves to emphasize the continued importance of email scanning and user awareness training. When the link or attachment is clicked, the malware writes its payload into the Windows registry and then disappears.

The payload written into the Windows registry contains a script hidden by several layers of tricks. The script is masked from view by removing the user’s access privileges or including a null character in the registry key name. The script calls a legitimate Windows program, such as PowerShell, to insert malicious code into the memory space of standard Windows processes, such as svchost, dllhost, or regsvr32, so that the code cannot be detected by scans for malicious processes.

Examples that McAfee Labs has seen so far, such as Kovter and Powelike, connect with websites and click-through ads, transforming the infected system into a click bot. The more resources the system has, the more ad traffic it generates, and the more money it makes for the criminals. Some analyzed variants have also downloaded ransomware payloads.

Fileless malware registry attacks leave no malware files to scan and no malicious processes to detect. Further, evidence of malicious registry keys is hidden by removing access controls and encrypting the values. Operating system updates and patches will not prevent attacks, because no vulnerabilities have been targeted. Specific rules are available that write-protect registry keys and processes, but more generic write-protection rules are not advised as many legitimate programs need to make registry changes.

The defense to this and other new types of fileless attacks is not just an updated virus definition file or even a single “silver bullet” tool. We need to look at user and system behavior, not individual devices, files, and processes. Humans are often the weakest defensive link, and this type of attack relies heavily on that vulnerability.

Email and web gateways can also help protect users from the initial malware. As soon as a malicious link is detected by a gateway, it should not only be shared with others, but a network traffic analysis should scan for recent traffic to the URL to identify computers that may be already infected. Because early emails may get through before the URL is convicted, endpoint security should watch for registry changes, as well as script-based execution of management tools like PowerShell and Windows Management Instrumentation. Hardware-based memory protection is an emerging security category that brings another layer to the defender’s side. Finally, behavioral analysis will help to quickly contain the infection by identifying infected computers shortly after automated ad clicking begins. Detecting the undetectable requires a team of defenses.